To max out your TSP under the current contribution limit ($18,000 for 2017) you would need to contribute $1,500 per month from your pay checks. That is very aggressive, but it is certainly possible, depending on your rank, pay grade and living expenses. It is important to note, employer contributions are in addition to the annual elective deferral limit. Visit TSP.gov for more information on contribution limits.

According to this FAQ, the match will go directly into Traditional TSP since Uncle Sam will want to get their taxes on the withdrawal end. The text from the document says:

Q2.7. Can a service member put their contributions into the Roth TSP? A2.7. Service members can elect to put their contributions into a traditional account, Roth account, or both. Service members can contribute in any whole percentage they choose subject to Internal Revenue Code (IRC) limits. Service members can make adjustments to their TSP account online at any time. All government automatic and matching contributions are based on the total amount of money (traditional and Roth) that the service member contributes each pay period. All government contributions are deposited into the service member’s traditional account.

Now I have also come across a conflicting article saying they are shutting down the limit to $2,400 a year. Not good news if true, but again it is just a proposal. Get on this new $18,500 limit and then $24,500 ‘catch up’ at age 50.